The present invention relates to a code converter circuit for digital signal transmission, and more particularly to a code converter circuit to be used for transmitting binary digital signals (hereinafter referred to merely as "digital signals") respectively through a pair of transmission paths having ambiguity. The term "ambiguity" is intended to mean that the terminal-to-terminal correspondence between the input and output sides of the paired transmission paths has the possibility of being reversed.
In case that two digital signals are transmitted through the paired parallel transmission paths having ambiguity, sometimes it is convenient that even if these transmission paths were to be equivalently interchanged the signals on the transmitter side could be reproduced on the receiver side without being interchanged. More particularly, it is desirable in the paired transmission paths having a pair of input terminals and a pair of output terminals that the signals applied to the respective input terminals can appear correctly at the corresponding output terminals, regardless of whether the input terminals are connected to the corresponding output terminals, respectively, that is, directly connected or the input terminals are connected respectively to the different output terminals in a crossed relation.
In order to realize this, it has been the practice to identify the ambiguity of the paired transmission paths from received signals, with the transmission paths crossed again on the receiver side, if necessary. Conventional methods for identifying at the receiving end the input terminals to which the respective digital signals are applied are exemplified as follows:
(1) On the transmitter side a pilot code signal is inserted into the digital signals, and on the receiver side the incoming pilot code signal is detected.
(2) On the transmitter side at first a test pattern signal is transmitted prior to transmission of main signals, and on the receiver side the received test pattern signal is detected. However, in the case (1) above there is a disadvantage that a device for speed conversion of transmission signals becomes complex. Also, in the case (2) above there is a disadvantage that main signals cannot be transmitted during the period when the test pattern signal is being transmitted. In addition, in case that the ambiguity of transmission paths is caused by accident, the maintenance of the transmission paths involves large amount of manpower and also the reliability is deteriorated.